A magnetic encoder is used, for example, for detecting a crank angle of an automobile. FIG. 13 shows an example of a conventional magnetic encoder. The magnetic encoder comprises a signal output part 101 used as a position standard. The signal output part 101 is arranged so that the width in a circumferential direction of a magnetic pole would be twice as long as that of a magnetic pole on another part in the circumferential direction. The width of all of magnetic poles in an area other than the signal output part 101 in the circumferential direction is arranged to be fixed (to be a half of the width in the circumferential direction of a magnetic pole in the signal output part 101). A rotational angle based on the signal output part 101 (0°) is detected in the area.
Further, there is also a technique disclosed in JP-A-7-74020. In the technique, the signal output part used as a position standard projects in a convex shape, while a predetermined arrangement pattern of magnetic poles is continued in the other parts. A rotational angle based on the signal output part is detected in the arrangement pattern part of magnetic poles.
In such a magnetic encoder, in order to detect a signal output part used as a position standard, the signal output part should pass through a part for detection by means of a sensor. Accordingly, the magnetic encoder is sometimes rotated at the maximum angle of nearly 360° for the purpose of detecting the signal output part.
Recently, an idling stop has been encouraged from an environmental point of view. Fuel is jetted to all cylinders, however, simultaneously with restarting an engine. The fuel jet to all cylinders is not preferable from an environmental point of view. In order to solve the problem, it is required to position a cylinder on the basis of a crank angle, a cam angle, or the like just after starting an engine to jet fuel only to a cylinder necessary to be fueled.
In the conventional case, however, a magnetic encoder should be rotated at the maximum angle of about 360° for the purpose of detecting the signal output part. Accordingly, a rotational angle cannot be specified with little rotation in a short time.
JP-A-9-49453 and JP-A-11-229948 disclose techniques in which the rotational angle can be specified in a short time. In the case of these techniques, however, structures are complicated and the number of components is large, and therefore, further improvement is desired.
An object of the invention, therefore, is to provide a magnetic encoder capable of specifying a rotational angle with little rotation.